This invention relates to an apparatus for continuous vacuum oil cleaning connected in parallel to an oil system, which apparatus is particularly suitable for continuous cleaning of oil fillings of high voltage transformers.
Known solutions for protection of oil fillings, particularly of oil fillings of electric transformers using oil as a dielectric and as cooling liquid are, in most cases, limited to a passive protection against contaminations from the surrounding atmosphere, combined in case of a dangerous increase of contaminating admixtures in the oil with a once performed cleaning process, achieving by external cleaning at a standstill of the machine. The typically passive protections of oil fillings act, in most cases, as a kind of isobaric closure, allowing a free passage of the surrounding air above the free level of oil in a conservator of the machine, while collecting selectively, the air moisture from the surrounding atmosphere, which air is supposed to be the main contaminanate of the dielectric liquid.
Most of known technical solutions by means of which the contamination of oil fillings of transformers by air moisture is limited, are based on freezing of water vapors in an air stream, flowing from the surrounding atmosphere to a conservator of the machine at a reduced load of this machine. The absorption capability of silicagel filters is also analogically utilized, which filters are similarly as in the preceding case introduced between the conservator of the transformer and the surrounding atmosphere.
The hermetization of transformers can be also included to passive protection methods of oil fillings. By a hermetic separation of the free oil surface in the conservator, from the surrounding atmosphere by an elastic diaphragm, or by a bag or by a thermodynamic closure, it is possible to limit very efficiently not only the infiltration of air moisture into the protective filling, but also prevents the contamination of oil by atmospheric oxygen.
Active methods of protection of oil fillings of electric transformers differ from the mentioned passive methods primarily in that the external contamination, process, due to the surrounding atmosphere, is not only slowed down by these processes, but they also enable an active removal of contaminations dissolved in oil so that the condition of the oil filling is not only maintained for a long time, but can also be improved.
Most known solutions of this kind are continuous methods of oil cleaning at substantially reduced pressure. These apparatus, generally called vacuum oil cleaners, can be mutually differentiated primarily by the method of which the required vacuum, causing subsequently the release of contaminating admixtures from the oil, is generated.
All mentioned methods for protection of oil fillings and their corresponding apparatus have some drawbacks. The main drawback of the passive protection of oil fillings, by means of freezing devices or by silicagel absorbers, is that they only enable to reduce the relative moisture of the air medium above the free oil level in the conservator. They prevent, thus, to a high degree, the saturation of oil by water, but do not influence the saturation of oil by gases contained in the air and also not by gases generated within the oil filling of the transformer in the course of operation conditions. The hermetization of oil in the conservator, on the other hand, prevents any access of air and water vapors to the oil filling, but prevents at the same time any, escape of gases generated in the oil filling of a transformer due to creation of so-called hot spots. The danger of a substantial saturation of oil by gas is particularly sufficiently known on high voltage electric transformers. It shows particularly, in case of a quick increase of the load of the transformer with a following increase of temperature of its oil filling, where due to a following oversaturation of the oil medium by gases, dissolved gases are released in the shape of bubbles. The creation of gas bubbles causes a substantial reduction of the electric strength in the oil dielectric similarly as an increased moisture content. In more favorable cases, the transformer is cut off from the mains by action of a gas relay, whereas in extreme situations, an electric breakdown within the machine occurs with substantial damage not only on the proper transformer, but also on the whole distribution system connected thereto. A lasting increased content of gases dissolved in the transformer oil has a major influence on the long time reliability of the machine. An already slightly increased content of oxygen in combination with moisture of the oil, cna cause of a quick aging both of the oil filling and of the insulating oil cellulose system.
The active methods of cleaning oil fillings in electric transformers have so far shown respective arrangements that are connected continuously to the machine which is in operation, and are capable to limit the occurrence, or at least of the frequency, of these mentioned failures. Their drawback is their complicated design, a high number of mobile parts, and a rather low specific performance compared with their size, input and weight. All of these mentioned drawbacks are overcome by the present apparatus for the continuous vacuum cleaning of oil.
The relevant part of these apparatuses apply solely to a quasistationary cleaning process on the basis of a forced diffusion generated due to a reduction of the overall pressure above the levels, surfaces, films or drops of the cleaned oil. The intensity of the process of forced diffusion on the boundary liquid gas is determined by concentration gradients, but at quasistationary processes, the contaminating gases and vapors cannot be released quickly enough even from a substantially oversaturated liquid medium either without sufficient occurrence of nuclear cores required for creation of bubbles or of suitably shaped hollows, where the bubbles can grow on the basis of surface forces. At rather viscous liquids, an example of which can be for instance a relatively cold transformer oil, the capillary forces onthe surface of spherical bubbles act against their growth and slow down a free release of bubbles through the surface of the liquid. The same type of capillary forces causes an increase of pressure within small drops of the cleaned liquid. So far a solution of increasing the overall interphase surface by atomizing has been used, but these concentration gradients reduce the interphase surface drop-vacuum, and again make impossible a quick diffusion of gases and vapors from the cleaned medium.
A certain increase of the separating effect of the above mentioned apparatuses is usually obtained by thin layers of the cleaned medium which flows down along suitably arranged built-in elements or inserts of vacuum chambers. This quantitative way for increasing the separating effect leads to an increase of the size of vacuum chambers. A once performed cleaning cycle is therefore also applied at the major part of apparatus where the time of retention of oil, in vacuum chambers, is limited to a relatively short time. If we consider the rather slow speed of separation of gas and vapors at the process of forced diffusion, it is impossible to avoid at the majority of actual apparatus that, the cleaned liquid, taken off from vacuum spaces, remains always oversaturated, i.e. insufficiently relieved of dissolved contaminants. In case of higher residual contaminations, of the taken-off liquid, it is also impossible to avoid another negative condition represented by cavitation of pumps, by means of which the cleaned liquid passes from vacuum spaces to spaces with normal pressure in the container of the transformer. However, not only the pump is exposed to cavitation processes. Bubbles released due to cavitation are together with oil forced back into the oil filling of the transformer. This again endangers the machine by electric disruption discharges. Another frequently used method for increasing the separating effect of the apparatus is an increase of temperature of the treated oil. Due to reduction of capillary forces, corresponding to the reduction of the viscosity of the liquid due to the increase of the temperature, it is possible to achieve a higher separating intensity. However, in the case of an increase of the oil temperature to 80.degree. to 100.degree. C. a rather quick degradation of natural inhibition agents of aging of the oil medium is experienced. In case this method is applied, the thus treated oil has to be on the output of the apparatus artifically inhibited.